The present invention is directed to a thermal flow-rate sensor for detecting a flow rate of a fluid, and in particular, to a signal processing method and apparatus intended to improve response to variations in a flow rate.
A well-known thermal flow-rate sensor of the prior art is a thermal air flow-rate sensor for detecting an air intake flow rate of an internal combustion engine. Such a sensor typically comprises a known hot wire type air flow-rate sensor adapted to obtain an air flow-rate signal corresponding to a current value by controlling an electric current supplied to a platinum wire to maintain the temperature of the platinum wire placed inwardly in an intake passageway constant. A thermal air flow-rate sensor in which platinum, instead of a platinum wire, is deposited on an aluminum substrate or a film has recently attracted much attention as an inexpensive version of the hot wire type air flow-rate sensor.
The thermal air flow-rate sensor is arranged to measure an air flow rate by detecting the electric current supplied to a temperature-dependent resistor controlled at a constant temperature. However, this arrangement gives rise to errors in detecting characteristics due to a lag in the output response when the air flow rate to be measured varies because of the temperature-dependent resistor, and heat accumulation and thermal conduction in a support member thereof. In the case of the hot wire type air flow-rate sensor wherein, e.g., a platinum wire is stretched inside the intake passageway, the influence of such a support member is relatively small. In the case of a hot wire type air flow-rate sensor including a platinum wire wound on a ceramic bobbin, or a thermal air flow-rate sensor wherein platinum is deposited on an aluminum substrate or on a film, however, the heat accumulation and thermal conduction in the film or the ceramic support member show an undesirable increase.
A known method of improving the foregoing defect, as disclosed in, e.g., Japanese Patent Public Disclosure No. 134919/1988, involves an amelioration in the characteristics of response to variations in air flow rate by adding a contrivance described in the publication to the structure of the thermal air flow-rate sensor.
There arise, however, problems inherent in the thus constructed thermal air flow-rate sensor, wherein the structure becomes intricate, on account of the fact that the response is improved by the sensor itself, to thereby cause a difficulty in manufacturing the sensor which in turn causes a rise in price.